Plasmas have gained a wide acceptance in materials processing with their application to etching and depositing layers on the surface of a substrate. More particularly, plasma sources are used in processing semiconductor wafers for use in the manufacture of semiconductor devices such as integrated circuit devices. Plasma sources have also been used in other material processing such as in the coating of flat panel displays. The basic properties required of a plasma for most processing applications are high density, low plasma potential, high uniformity of plasma, and operation over a wide pressure range. A low pressure, high density plasma is superior at etching and depositing layers on the fine scale that microelectronic and other applications require. Additionally, to produce consistent results over the area of the substrate being processed the plasma must have a uniform density over the area of the substrate being processed.
Greater efficiency in processing substrates can be achieved by increasing the area on a substrate capable of being processed. To reduce the cost of production a plasma processing device would ideally be able to process as large a substrate as possible. Thus, an essential feature of any successful plasma processing device is the ability to scale up the source chamber to be able to efficiently process larger areas of a substrate.
It is well known in the art that a plasma can be generated in a low pressure gas, such as argon or SF.sub.6, by exciting helicon waves with the use of a radio frequency (hereinafter RF) antenna. Helicon wave plasma etching has advantages over conventional parallel plate, ECR (electron cyclotron resonance) or RFI (radio frequency inductive) plasma systems. For instance helicon wave plasma etching requires a lower magnetic field than ECR and helicon discharges have a lower plasma potential than parallel plate etching. To excite helicon waves in an ionized gas within the source chamber energy from the RF antenna is coupled into plasma to produce the helicon waves. A helicon wave is an electromagnetic wave which, in a source chamber with a cylindrical geometry, will propagate along the magnetic field lines of a magnetic field in the source chamber. A source chamber with cylindrical geometry allows the propagation of both right- and left-hand polarized helicon waves. Conventional cylindrical geometry source chambers therefore allow the propagation of helicon waves and the resulting plasma out of the source chamber and into a vacuum chamber for processing a substrate. This was the geometry used by Boswell in U.S. Pat. No. 4,810,935, the subject matter thereof being incorporated by reference.
While cylindrical geometry source chambers are capable of producing a uniform, high density, low pressure plasma which can be propagated along magnetic field lines into a vacuum chamber by helicon waves, the conditions that allow helicon wave propagation, specifically cylindrical geometry, pose significant limitations on the scaling up of the source chamber to allow the processing of larger area substrates. Since helicon plasma waves characteristically produce high density, low pressure plasmas ideal for processing substrates it is advantageous for any source chamber geometry to be able to use helicon waves to create and propagate the plasma from the source chamber to the substrate. Considerable research in the area of antenna design has led to significant improvements in the area, density and uniformity of plasma generated with cylindrical geometry source chambers. Despite these improvements, significant limitations remain that make scaling up a cylindrical geometry helicon wave plasma source chamber impractical. Accordingly, there is a need in the art for a plasma processing device with a scalable plasma source chamber capable of producing a uniform, high density, low pressure plasma which can be propagated from the source chamber into a vacuum chamber for processing a substrate.